This invention relates to a filter, and a method of operating a pass filter, in which the cut-off frequency can be stepped down gradually.
High-pass filters with variable cut-off frequency are well-known. For example, a high-pass filter may include a capacitor in line with the signal being filtered, and a resistance between that capacitor and ground. A variable high-pass filter of that type may have several resistances in parallel, each capable of being switched in or out of the circuit. The cut-off frequency is determined by which of the resistors is switched in.
It has been observed that under certain conditions, changing the resistance abruptly to change the set cut-off frequency can cause undesirable transient effects. For example, such a filter may be used as an input filter network on the data or servo channel of a disk drive controller. It has been observed that changing the cut-off frequency abruptly can give rise to false signals. In the case of a disk drive controller, this is particularly the case when operating in perpendicular recording mode.
It would be desirable to be able to change the cut-off frequency of a filter without giving rise to false signals.